Application of 2D pulses has been shown to be advantageous for EPI applications such as DWI with a reduced FOV in the phase encoding (PE) direction [1-4]. In this method, multiple thin slice profiles are selected, which are limited along two encoding directions of the 2D excitation trajectory and extend across the imaged object in the non-encoded dimension. Recently, parallel excitation (PEX, [5,6]) has been demonstrated for shortening 2D EPI-encoded pulses [7-9]. However, application of PEX to EPI encoding of limited profiles was found suboptimal, since the undersampling artifacts could not be completely eliminated [7], especially for high flip angles. In this work, the main goal was to investigate the feasibility of 2D PEX of limited profiles encoded by spiral trajectories. For this purpose, we exploited the highest possible undersampling factors. Finally, we analyzed the benefits and confounding conditions of 2D PEX of limited slices encoded by spiral and EPI trajectories.

In contrast to EPI, the resolutions along the encoding directions of a spiral trajectory are related to each other (Figure 1A). The undersampling artifacts for spirals are represented by swirls and streaks, having little similarity with the original profile. This effect is due to the fact that undersampling of the spirals leads to a reduction of the field of excitation (FOE) along all spatial directions around the excited voxel and therefore the sample is “aliased” into itself in all radial dimensions. The undersampling artifacts for EPI are seen as replicates of the main profile and confined to a narrow location in space if a thin profile was defined (Figure 1B). In PEX, the sensitivities of multiple transmit elements are used during 2D pulse calculation [5] and the undersampling artifacts are eliminated during excitation. In this manner, artifacts due to small errors and variations in the B₁ data would be blurred over the object for spirals and their intensity is minimized substantially compared to EPI trajectories.

Experiments were conducted on a 3T MR scanner (Siemens Magnetom TRIO) with an 8-channel TxArray extension. Slice profiles encoded by spiral trajectories were excited in a phantom, which was a Plexiglas container filled with water solution of 1g/L CuSO₄ and 0.9 g/L NaCl. The phantom size was 20×25×30 cm³ and the selected profiles were imaged using 3D GRE method and 2D SE-EPI. The scanning parameters were: 1) excitation flip angle = 90°, FOV = 25.5×25.5 cm² and matrix = 128×128 for SE-EPI; 2) excitation flip angle = 30°, FOV = 25.5×19.2×28.8 cm³ and matrix = 128×96×48 for GRE.

The spiral excitation trajectories were undersampled by factors 2 and 4 (Figure 2). They were defined on a grid size of 64×64 samples over the FOE of 38.4×24 cm² and the resulting slice thickness was 12 mm. The 2D pulses were calculated with an iterative optimization method using conjugate gradients for the small-tip-angle approximation. Relative axial B₁ sensitivities of the transmit coil and an axial B₀ map were used in pulse computations [10].

The selected profiles for pulses with the undersampling factors of 2 and 4 are demonstrated in Figures 3 and 4, respectively. Undersampling of excitation trajectories led to excitation of swirls and streaks re-distributed over the originally defined FOE as indicated by white arrows in Figures 3A and 4A. Taking into account B₁ maps allowed for suppression of the artifacts to a negligible level comparable to the image noise. It is important to note that the selected profiles appeared to be clean from the artifacts along the third non-encoded dimension of the trajectory. Furthermore, a good precise profile selection was observed for both excitation flip angles of 30° and 90°.

In comparison to previous studies on 2D EPI-encoded parallel excitation of limited profiles [7,8], spiral pulses are more robust against B₁ errors in the calibration data, which is due to the “spatial incoherence” of the undersampling artifacts. Another property making spirals attractive for PEX is that they exhibit slight oversampling at the k-space center in comparison to peripheral locations, providing additional encoding information and thus improving further their robustness against B₁ deviations. Nonetheless, spiral pulses are still relatively long for encoding thin profiles with a thickness required for clinical applications despite of the demonstrated shortening with PEX. A remedy could be accomplished by undersampling spiral trajectories for higher factors, which would require using MRI systems with a number of transmit channels more than eight [11].